The present invention relates to a method and apparatus for determining the amount of deviation, or arc of bending, of an electrode wire stretched and being longitudinally displaced between two guide members, in the course of a cut being effected in an electrode workpiece by electro-erosive discharges.
In the course of cutting a workpiece in a travelling wire EDM apparatus, the electrode wire, whose diameter is of the order of 0.25 mm., for example, is subjected to forces of repulsion and attraction which cause the portion of the wire in the machining zone to adopt a curvilinear shape instead of an ideal rectilinear shape. The forces of repulsion result from the gases being released by the electrical discharges, and the forces of attraction, electro-magnetic and electro-static, are caused respectively by the current and the voltage of the machining pulses. While effecting a cut, the forces of repulsion are paramount and the electrode wire tends to lose its alignment with its guide members until the wire takes a curved shape resulting from an equilibrium between the forces acting on the wire and the longitudinal pull due to the mechanical tension exerted on the wire by the wire feeding mechanism.
It is known that the deflection of the wire between its guide members causes important machining errors, because every time the direction of the cut is changed, the curved portion of the wire lagging behind a straight line connecting the guide members follows a trajectory having a shorter path than the guide member trajectory. It is therefore very important to determine the amount of curvature of the wire, and more particularly the vertex length or height of the arc of curvature of the wire such as to apply known method for correcting, for example by slowing down the machining speed or by modifying the guide member trajectory.
A known method for measuring the amount of deviation of the wire, as disclosed in U.S. Pat. No. 4,521,662, consists in directly measuring, by optical or electrical means, the amount of deviation of the wire relative to its ideal rectilinear shape. The measuring is effected between the surface of the workpiece and one of the guide members. However, the method is complicated and utilizes a cumbersome device located in a portion of the EDM apparatus where room is limited, more particularly when the guide members are very close to the workpiece, which is generally the case as it is desirable to locate each of the wire members in close proximity to the workpiece, in order to limit the amount of bending of the wire between the guide members.